Generally, to automatically inspect a printed wiring board pattern, U.S. Pat. No. 4,152,723 granted to D. H. McMahon et al., on May 1, 1979, discloses using a helium-cadmium, blue laser beam that is swept by a rotating mirror across a portion of the printed wiring board surface. The copper conductors of the printed wiring board reflect the blue light, but the insulating substrate surface fluoresces orange in response to the blue laser beam. The light reflected from or fluoresced by the printed wiring board passes through a blue-blocking filter onto a photomultiplier tube. The output of the photomultiplier tube is then detected as a binary representation pattern of the presence or absence of copper along the line of spots over which the laser beam sweeps, thereby eventually generating an accurate representation of the printed wiring board's surface.
U.S. Pat. No. 4,185,298 granted to Billet et al., on Jan. 22, 1980, discloses the automated inspection of patterns such as a printed circuit board by scanning the test board and a master or standard board known to be accurate. The scanning is done with a pair of TV cameras, the outputs of the cameras are compared, and an error signal is produced when the outputs do not coincide.
U.S. Pat. No. 4,338,626 granted to J. H. Lemelson on July 6, 1982, discloses a system for comparing a video signal with a reference or standard signal that is recorded on magnetic tape or on a magnetic drum.
U.S. Pat. No. 3,835,249 granted on Sept. 10, 1974, to A. J. Dattilo et al., discloses a system for indicating the precise position of a scanning light beam (scanned by a rotating multifaceted mirror). The scanning light beam is split into two paths by a beam splitter. The two paths comprise a utilization path and a synchronization path. The portion of the light beam traversing the synchronization path scans an optical grating. The light passing through the grating is detected to provide an output signal which can be used to indicate the position of the scanning beam traversing the utilization path. In this way, a binary signal can be used with subsequent electronics, such as a binary counter, to identify or control a video signal at actually known positions along the sweep of the beam.
As the above references illustrate, in the manufacture of printed wiring boards, it is desired to inspect the printed wiring pattern on the board in order to discover and possibly correct any defects in the copper pattern which might cause operational difficulties in the finished printed circuit. Typically, these defects comprise nicks in the edges of a copper path which may actually cause a break in the circuit, or enlargement of the copper path so as to diminish the spacing between adjacent copper conductors sufficiently to cause bridging and resultant conduction between the paths. With the ever-increasing size of printed wiring boards and the decreasing conductor size and spacing, manual inspection of a printed wiring board approaches a practical impossibility.
As disclosed in the above-mentioned McMahon patent when a beam of blue laser light energy strikes a treated fiberglass-epoxy substrate, the substrate fluoresces orange. The orange light is then filtered to remove any blue component and is passed to one or more photomultiplier tubes in order differentially to sense the copper covered areas of the printed wiring board and distinguish them from the exposed areas of the substrate. As disclosed in the McMahon reference, the beam of laser light is preferably swept across a portion of the printed wiring board by means of a rotating mirror. Typically, such swept systems, which have been known since before the invention of the laser, usually produce the resultant data in short bursts punctuated by long pauses between bursts of data.
It is rare that the sweep across a reasonable length of a printed wiring board can be accomplished at a uniform speed. Consequently, typically, the speed of sweep varies from one end of the sweep to the center of the sweep to the opposite end of the sweep. However, it is important to sample the data at uniform length spacings of the swept beam. This is to avoid distortion of the electronic representation of the surface being swept.
An important factor in the inspection of a printed wiring board is the uniformity of width of the copper conductors. If the electronic representation shows a conductor wider at the end of the beam's sweep than another conductor at the center of the beam's sweep, an error signal might result. Conversely, an improperly sized conductor could pass inspection merely by reason of its location within the length of the sweep.